Rolling bearings are assembled in the rotation support sections of various kinds of machinery including vehicles, machine tools, industrial machinery, and other general machinery. Moreover, ball screw apparatuses may also be assembled in various kinds of machinery as an axial feeding apparatus, and as members of such rolling bearings and ball screw apparatuses, there are bearing ring members that have a raceway surface formed completely around at least one circumferential surface of the inner-circumferential surface and outer circumferential surface. FIG. 14 illustrates an example of conventional construction of a rolling bearing unit for wheel support as disclosed in JP 2012-006017(A), and is a rolling bearing unit for supporting wheels of an automobile, and a rotating member for a brake such as a brake disc or the like so as to be able to rotate freely with respect to a suspension.
The rolling bearing unit for wheel support illustrated in FIG. 14 is for a driven wheel (front wheel in a FR or MR automobile, and a rear wheel in a FF automobile), an outer ring 2, a hub 3 and plural rolling bodies 4. The outer ring 2 has a double row of outer-ring raceways 5a, 5b formed at two locations in the axial direction of the inner-circumferential surface, and a stationary-side flange 6 formed in a portion near the inside end in the axial direction of the outer-circumferential surface (here, “inside” in the axial direction refers to the center side in the width direction of the vehicle when assembled in an automobile (right side in FIG. 14), and the “outside” in the axial direction refers to the outside in the width direction of the vehicle (left side in FIG. 14)). Moreover, the hub 3 is constructed by combining and fastening together a main hub unit 7 and an inner ring 8 using a nut 9, and has a rotating-side flange 10 that is formed on a portion near the outside end in the axial direction of the outer-circumferential surface of the hub 3, and double-row inner-ring raceways 11a, 11b are formed at two location in the axial direction of a portion in the middle section and a portion near the inside end in the axial direction of the outer-circumferential surface of the hub 3. Plural rolling bodies 4 are arranged in each of the double rows between the inner-ring raceways 11a, 11b and the outer-ring raceways 5a, 5b, so that the hub 3 can rotate on the inner-diameter side of the outer ring 2. In the operating state, the stationary-side flange 6 of the outer ring 2 is joined and fastened to the knuckle of a suspension apparatus, and the wheel and brake rotating member are joined and fastened to the rotating-side flange 10 of the hub 3.
The outer ring 2, main hub unit 7 and inner ring 8 of the rolling-bearing unit 1 for wheel support have a raceway surface formed around the inner-circumferential surface or outer-circumferential surface thereof, and correspond to a bearing ring member. This kind of bearing ring member is made by forging a metal raw material, and then performing finishing processing such as a machining and polishing. As an example of a conventional manufacturing method for a bearing ring member is a manufacturing method for an outer ring 2 as disclosed in JP 2012-006017 (A), which will be explained with reference to FIG. 15A to FIG. 15F.
In order to manufacture the outer ring 2, first, as illustrated in FIG. 15A, a metal circular column shaped raw material 13 is used. The size of the raw material 13, depending on the type of bearing ring member to be manufactured, is normally such that a ratio of the diameter and length in the axial direction is about 5:4 to 5:6. In the first upsetting process, the upset intermediate material 14 is obtained by crushing the raw material 13 in the axial direction as illustrated in FIG. 15B. The upset intermediate material 14 is such that the outer diameter of the middle section in the axial direction becomes larger than the outer diameter of both end sections in the axial direction, so as to have a beer barrel shape or thick disk shape. The length in the axial direction of a beer barrel shaped upset intermediate material 14 is about 40% to 70% the length in the axial direction of the raw material 13, and the length in the axial direction of a thick disk shaped upset intermediate material is about 30% to 35% the length in the axial direction of the raw material 13.
Next, in a preforming process, the perimeter of the upset intermediate material 14 is surrounded by preforming die, and the upset intermediate material 14 is plastically deformed by pressing the center section of both end sections in the axial direction of the upset intermediate material 14 with a preforming pressure punch. Then, a preformed intermediate material 15 as illustrated in FIG. 15C is obtained. The preformed intermediate material 15 has a pair of open concave sections 16a, 16b on both end surfaces in the axial direction of the preformed intermediate material 15, a partition section 17 that is located between the bottom sections of the concave sections 16a, 16b, and a stationary-side flange 6 that is formed so as to protrude outward in the radial direction at a position near one side in the axial direction of the outer circumferential surface (top side in FIG. 15C). The thickness in the axial direction of the partition section 17 of the preformed intermediate material 15 is about 15% to 30% the length in the axial direction of the upset intermediate material 14, and the thickness in the radial direction of the cylindrical portion 18 of the preformed intermediate material 15 is about 15% to 25% the diameter of the upset intermediate material 14.
Next, in the finish-formation process, a finish-formation die is placed around the preformed intermediate material 15, and in this state, a pair of finish-formation pressure punches press both end surfaces in the axial direction of the preformed intermediate material 15. As a result, the partition section 17 is crushed in a direction such that the thickness dimension is reduced, and the shape of the cylindrical portion 18 that is located around the partition section 17 approaches the shape of the outer ring 2. Then, the finish-formed intermediate material 19 illustrated in FIG. 15D is obtained. The thickness in the axial direction of the partition section 17 of the finish-formed intermediate material 19 is about 10% to 20% the length in the axial direction of the upset intermediate material 14, and the thickness in the radial direction of the cylindrical portion 18 of the finish-formed intermediate material 19 is about 10% to 20% the diameter of the upset intermediate material 14.
Furthermore, in the punching process, by punching and removing the partition section 17 of the finish-formed intermediate material 19 except the portion around the outer perimeter edge, a punched intermediate material 20 as illustrated in FIG. 15E is obtained. Finally, in a deburring process, by removing burrs 21 that remain on the outer perimeter edge section of the stationary-side flange 6 of the punched intermediate material 20, a final intermediate material 22 as illustrated in FIG. 15F is obtained. After that, a finishing process such as machining and polishing is performed on each part of the final intermediate material 22, to complete the outer ring 2 illustrated in FIG. 14.
Incidentally, the column shaped raw material 13 illustrated in FIG. 15A is obtained by cutting a long member that was extrusion molded by a steel manufacturer, having a circular cross section with respect to a virtual plane that runs in the axial direction, and it has been conventionally known that the composition (cleanliness) of the raw material 13 is not uniform. Moreover, it is known that when the metal portion of the raw material 13 having a large amount of oxide-based non-metallic inclusions and low cleanliness is exposed to the portion of the double-row outer-ring raceways 5a, 5b that are formed around the inner-circumferential surface of the outer ring 2, and particularly the portion that comes in contact with the rolling surfaces of the rolling bodies 4, it becomes difficult to maintain the rolling fatigue life of that portion.
In the manufacturing method disclosed in JP 2010-006017 (A), the forging process is designed so that a portion where the amount of oxide-based non-metallic inclusions is small and the level of cleanliness is high that exists within a range of 50% to 70% the radius from the center of the raw material 13 can be located in the outer-ring raceways 5a, 5b of the outer ring 2. As a result, it is possible to manufacture at low cost an outer ring 2 for which the rolling fatigue of the outer-ring raceways 5a, 5b can be sufficiently maintained. However, in order to cause the portion of the raw material 13 having a high degree of cleanliness to flow in the outer-ring raceways of the outer ring during the forging process, there is a possibility that the forging process, and the construction of the outer ring to be manufactured, for example, the outer-ring raceways or the position of the stationary-side flange will be limited, and thus the freedom of design will be limited. In other words, when manufacturing the raw material, the portion having a high level of cleanliness is regularly in nearly the same position, so in order for the forging process to cause the portion having a high level of cleanliness to flow to the raceway surface, the forging process, the shape of the bearing ring member and the like are restricted.
Moreover, as a main cause for reducing the rolling fatigue life of the raceway surfaces of the bearing ring member, in addition to the cleanliness of the raw material, there is the hardness and toughness of the metal material of the raw material. In case of forming the raceway surfaces of the bearing ring member, such characteristics affect the fatigue strength and wear resistance of the raceway surfaces. Therefore, as the raw material, using a simple metal material such as high carbon chromium bearing steel (SUJ2 to SUJ5) or the like having excellent fatigue strength and wear resistance has been considered for forming the raceway surfaces, however, this kind of metal material is very expensive when compared with steel for machine structures, so using this material for the entire bearing ring member would cause an increase in the material cost of the bearing ring member. Moreover, metal material such as high carbon chromium bearing steel has higher hardness and lower machinability when compared with steel for machine structures, so there is also the possibility of high processing costs for machining that is performed after forging.